1. Field of the Invention
This invention relates to a cobalt-base alloy having excellent high-temperature strength and high-temperature ductility and, more particularly, to a nozzle of a gas turbine, made of a casting of the cobalt-base alloy.
2. Description of the Prior Art
Conventionally, cobalt-base alloys have been used, e.g., for first-stage nozzles of a gas turbine which undergo rapid repetition of heating and cooling. The intended service life of the nozzles is 20,000 to 30,000 hr or longer at a temperature as high as 800.degree. to 1,000.degree. C. Such a cobalt-base superheat-resistant alloy has been produced by precision casting, and the development thereof has been directed mainly towards an improvement in a high-temperature strength, particularly an improvement in creep rupture strength. This unfavorably led to a disadvantage of the cobalt-base alloy that the high-temperature ductility is unsatisfactory. In fact, the examination on cracking caused in service revealed that the cracking was not attributable to a high-temperature strength but thermal fatigue due to repeated occurrence of thermal stress. Conventional cobalt-base alloys have a sufficient creep rupture strength as well as a creep rupture ductility enough to be put into practical use up to 900 .degree. C. However, the ductility lowers rapidly at a temperature exceeding 900.degree. C., e.g., 982.degree. C. Particularly, the results of a long-term creep rupture test over 1,000 hr or longer showed that the elongation was remarkably lowered and as low as several %. This suggests that, when a gas turbine is used at a temperature of 900.degree. C. or above, the thermal stress of the nozzle is a causative factor of occurrence of cracking. The material for the nozzle is required to have a combination of an excellent high temperature strength with an excellent high-temperature ductility.
The reason why conventional cobalt-base alloys are high in high-temperature ductility at a temperature of 900.degree. C. or below but rapidly lowered when the temperature exceeds 900.degree. C. is that cobalt in itself is generally low in oxidation resistance, which requires the presence of a high content of chromium, leading to formation of nonmetallic inclusions in the band form, which seem to be oxides, at grain boundaries during casting, which in turn makes grain boundary deformation difficult.
When the temperature is 900.degree. C. or below, the ductility of the alloy is high, because the ductility of the matrix is high due to formation of a small amount of precipitates and, at the same time, the extent of the influence of the nonmetallic inclusions on the grain boundaries is small. On the other hand, when the temperature is 982.degree. C. or above, carbides precipitate in the matrix to strengthen the latter, thus making it difficult to deform the matrix, which gives an influence on the grain boundaries.
When the amount of chromium is large, nitrides precipitate at a high temperature, e.g., 982.degree. C., which is a causative factor of lowering in the ductility of an alloy. Further, a cobalt-base alloy having a high content of chromium brings about a grain boundary oxidation at a high temperature, leading to lowering in ductility. In a high strength cobalt-base alloy, a solid-solution strengthening element (e.g., tungsten or molybdenum) and carbon are added to form a carbide, which strengthen the cobalt-base alloy. Such a carbide is often formed in a net-like shape. The carbide is selectively oxidized at a high temperature with ease. Therefore, when the oxidation atthe grain boundaries proceed, the resulting oxide brings about a stress concentration with respect to a tensile stress, which leads to lowering in strength and ductility.